1. Field of the Invention
The present invention relates to a light scanning apparatus and an image forming apparatus using the same.
More specifically, the present invention is suitable for an image forming apparatus such as a laser beam printer and a digital copying machine having an electrophotographic process in which image information is recorded by carrying out light scanning on a surface to be scanned with a light beam (laser beam) emitted from a light source means via an imaging optical system.
2. Related Background Art
Conventionally, as the light scanning apparatus used for the image forming apparatus, an optical system in which the light beam is made incident at an angle not orthogonal to a rotational axis of a deflection means (light deflection device) (diagonal incident light scanning apparatus) is known.
For example, in an over filled light scanning apparatus (OFS optical system), in many cases, the light beam is made incident on the light deflection device from a side of an imaging optical system (scanning lens system) in a sub-scanning direction with a certain angle so that a distribution of light amount is not asymmetrical in the left and the right.
Besides, in some cases of the light scanning apparatus having a smaller number of the light deflection devices than a number of the surfaces (photoconductive drum surfaces) to be scanned, a plurality of light beams is made incident on the light deflection device with different angles in the sub-scanning direction in order to separate the plurality of light beams deflected by the light deflection device.
In such a light scanning apparatus, it is difficult to always continue to make the light beam (ray) incident on the main scanning section including an optical axis of an optical surface provided to the imaging optical system.
This is because the light beam deflected by the light deflection device changes an advancing direction of the light beam in a cone-like shape. In addition, because a scanning line curvature occurs in accordance with the change in the advancing direction of the light beam, conventionally, the scanning line curvature is alleviated by positively decentering the optical surface.
Besides, in recent years, various types of the light scanning apparatuses provided with the imaging optical system and the like having a diffracting surface in order to carry out a temperature compensation are proposed. The light scanning apparatus like this is in the power arrangement in which the fluctuation of a positive refractive power at the time of fluctuation of an environmental temperature is set off by the fluctuation of a positive diffraction power at the time of the fluctuation of the environmental temperature (Japanese Patent Application Laid-open No. H10-333070 (U.S. Pat. No. 6,067,106)).
In such a light scanning apparatus, an imaging position is deviated in a direction of the optical axis when a wavelength is fluctuated by a mode hop (mode hopping) regardless of the environmental temperature.
Because of this, it is necessary to sufficiently secure a tolerable depth width. The light scanning apparatus in which the tolerable depth width is sufficiently secured is proposed (Japanese Patent Application Laid-open No. H09-054263 (no corresponding foreign patent application made)).
Incidentally, in the light scanning apparatus in which the imaging optical system is provided with the diffracting surface, if an optical element that configures the imaging optical system is decentered in a sub-scanning direction, when the wavelength of the light emitted from a light source is fluctuated, the imaging position is deviated in the sub-scanning direction, as well as in the optical axis direction.
Because of this, there arises a problem such that if the mode hop occurs during the scanning (while forming an image on the surface to be scanned by the light beam emitted from the light source means), image quality is deteriorated because of fluctuation of a scanning line interval before and after the occurrence of the mode hop.
Especially, in a color image forming apparatus, it is especially necessary to alleviate a positional displacement of the scanning line because the occurrence of the mode hop causes a color drift.